Wireless communication is an increasingly popular means of personal communication in the modern world. People are using wireless networks for the exchange of voice and data as an alternative to using a wire infrastructure. In principle, a user can seek information over the Internet or call anyone over a Public Switched Telephone Network (PSTN) from any place inside the coverage area of the wireless network.
With the increase of the popularity of wireless communications, many of the uses for a wire infrastructure are being replaced by wireless infrastructures. For example, a traditional wire infrastructure PBX may be replaced by a wireless PBX to route phone calls and Internet connections in an office. A wireless PBX may take advantage of wireless communication techniques.
A wireless communications technique commonly used to allow multiple users on the same channel is code division multiple access (CDMA). CDMA permits multiple users to use the same wireless communication channel at the same time, which allows the network infrastructure to support more wireless users. Some of the benefits of CDMA are improved call quality, simplified system planning through the use of the same frequency in every sector of every cell, enhanced privacy, improved coverage characteristics, increased talk time for portables, and increased bandwidth.
CDMA utilizes the radio spectrum by allowing multiple users to share the same physical channel. In CDMA, multiple users occupy the same frequency at the same time. Consequently, frequency and time are not used to discriminate between users. Instead, CDMA separates multiple users using the same channel through the use of codes. The receiver in the CDMA system typically receives a waveform that consists of a mixture of signals from several users. The system then uses coding to discriminate between the signals received from the multiple users on the same frequency channel at the same time.
CDMA is a type of spread spectrum communication technique known as Direct Sequence Spread Spectrum (DSSS). In this approach, a narrowband data signal from a user is spread through the use of a broadband code that is unique to the user in order to create a broadband signal for transmission. The broadband signal is then transmitted on a frequency that may also be used by other users. When a receiver receives the broadband signal, it uses the user's unique code to recover the user's narrowband data signal from the mixture of signals encountered by the receiver.
In CDMA, each bit of the user's narrowband signal is divided into a number “m” of short intervals called chips. Each bit is typically broken down into from 64 to 128 chips. Each transmitting user is assigned a unique chip pattern or sequence that is, in effect, that user's code channel. Using this unique sequence, the user's transmitted signal will be distinguishable by a receiver from other signals using the same physical channel. Other user's code patterns will appear random to the receiver and will integrate in a random self-canceling fashion such that they do no disturb the bit decoding decision being made with the selected user's code pattern.
Since each bit is represented by “m” chips, the information to be transmitted is increased from “b” bits to “mb” chips per second. This takes advantage of the high bandwidth in a CDMA system. Each chip may be represented by a complex number in Euclidean signal space. The real, or in-phase, component of a signal is modulated by a sine wave and transmitted at the operating frequency. The imaginary, or quadrature, component of the signal is transmitted with a 90° phase shift of the same wave.
A typical receiver used in CDMA receives multiple versions of the same signal, each version having a different delay. The multiple versions, or multi-path signals, may arise, for example through multiple paths due to reflections or from multiple transmitters. It may be difficult for the receiver to resolve a desired signal from apparent stronger signals received. One way to capitalize on this apparent disadvantage is to use a rake receiver. A rake receiver combines the multi-path signals with the main signal in order to increase the strength of the main received signal instead of degrading it.
The rake receiver attempts to gather as much signal power as possible by identifying the multi-path replicas of the transmitted signal and assigning separate correlators to each of them. These correlators are commonly referred to as the rake “fingers” and a rake receiver typically has three fingers. For each frame of received data signals, the receiver uses the combined output of the three rake fingers. Each finger may be configured to independently recover a particular code. However, the fingers may also be targeted on delayed multi-path reflections or on different transmitter stations.
A rake receiver captures the different time arrivals of a desired signal separately by exploiting the correlation properties of the spreading code used in a CDMA system. Typically, a rake receiver may be able to resolve signals that are at least ⅛ chip apart from each other. The signals that correspond to ⅛ chip apart from each other have been received 80 milliseconds apart from each other. To exploit the multi-path resolution possibilities via a rake receiver, the delay spread of incoming signals should be greater than the chip rate used in transmission. Therefore, conventionally, the fingers of a rake receiver are equally spaced from one another by ⅛ chip. In other words, each finger is delayed by 80 milliseconds from the adjacent finger.
In a CDMA distributed antenna system, a number of antenna elements at different locations may simultaneously communicate on the same frequency and same channel (e.g., same Pseudonoise (PN) Offset and same Walsh code) with a given mobile station. A Walsh code contains 64 sequences, each 64 chips long. Each Walsh code is orthogonal to all other Walsh codes. A Walsh code's orthogonal nature prevents one code from interfering with other Walsh codes. A common base transceiver station (BTS) may power the group of antennas used in the CDMA system. On both the forward link (communications from the base station to the mobile station) and reverse link (communications from the mobile station to the base stations), the physical separation of the antenna elements can give rise to phase differences and consequent signal distortion (e.g., cancellation, etc.).
With an existing distributed antenna system, the signals may get offset in phase by the time the signals reach the MS (mobile station). This may cause multiple problems. The orthogonal nature of the Walsh codes is lost when they are not time aligned. As a result, a loss of time alignment can cause interference among the various forward link traffic channels. Interference problems can also occur when there is a phase misalignment between the transmitted pilot and other Walsh channels. Interference may hinder the possibility of successfully receiving the transmitted signal.
A CDMA forward link system uses separate and distant in phase (I) and quadrature (Q) pseudorandom-noise (PN) spreading sequences. Any phase misalignment between the receiver and its incoming Walsh channel results in interference between the spreading sequences. Phase errors are essentially a loss of orthogonality between I and Q. These phase errors can result from crosstalk between I and Q in the base station baseband processing section, misaligned local oscillators, intermodulation (IM) between Walsh codes, or signal distortion due to the physical separation of the transmitting antenna elements.